FIG. 1 is a partial cross-sectional view showing a tank device 1 as an example of a conventional gas-pressure device. The tank device 1 is configured so that a valve unit 4 is attached to an opening portion 3 of a tank body 2, and gas is contained in a container space 5 of the tank body 2 and is exhausted from the container space 5 by controlling the valve unit 4. An inner thread is formed in the opening portion 3 of the tank body 2, and an outer thread is formed in the valve unit 4. The valve unit 4 is threadedly coupled with the opening portion 3 of the tank body 2. A seal member 6 is provided between the opening portion 3 of the tank body 2 and the valve unit 4.
A tip-end portion 3a of the opening portion 3 of the tank body 2 and a contact portion 4a of the valve unit 4 are in contact with each other in the axial direction. The valve unit 4 is threadedly coupled with the opening portion 3 with a tightening torque such that a tightening force Fc mutually acts on the tip-end portion 3a and the contact portion 4a. In such a configuration, if the tightening force Fc decreases, surface pressure acting on the contact surfaces 3a and 4a that prevents a rotation of the valve unit 4 with respect to the tank body 2 also decreases. Accordingly, as vibrations and impulse forces act on the device, the valve unit 4 may rotate with respect to the tank body 2, and the threaded engagement between the opening portion 3 and the valve unit 4 may then be loosened.
Factors that cause the decreasing of the tightening force Fc are, for example, a pressure of the gas contained in the container space 5, and a thermal expansion of the tank body 2 and the valve unit 4 due to a change in the ambient temperature ΔT. A pressing force Fp caused by the pressure of the gas contained in the container space 5 acts in a direction to which the valve unit 4 threadedly moves back with respect to the tank body 2, thereby decreasing the tightening force Fc. Further, where the tank body 2 and the valve unit 4 are made of materials that have a different coefficient of thermal expansion, when the ambient temperature changes, the tightening force Fc changes in accordance with the temperature change ΔT and the combination of the materials of the tank body 2 and the valve unit 4.
In order to prevent the loosening of the threaded portions, it is necessary to keep the tightening force Fc from reaching zero even when the pressure force Fp acts and the tank body 2 and the valve unit 4 thermally expand due to the ambient temperature change ΔT. Therefore, upon threadedly coupling the valve unit 4 with the tank body 2, an initial tightening torque should be given so that an initial tightening force Fc satisfies the following Equation (1).Fc>Fp+F{f(ΔT)}  (1)
Here, F{f(ΔT)} is an amount of decrease in the tightening force Fc due to the ambient temperature change ΔT. Since the tank device 1 does not include a configuration that positively prevents the loosening of the threaded portions, the valve unit 4 is threadedly coupled with the tank body 2 with the initial tightening torque such that the tightening force Fc is large enough to satisfy the Equation (1).
Japanese Examined Utility Model Publication No. SHO 46-8496 discloses a non-loosening nut that is a combination of a nut and a washer formed from an elastic plate. The non-loosening nut is configured so that the elastic washer is partially pressure-bonded to a thread of a bolt to avoid the loosening of the nut. Further, Japanese Laid-open Patent Application No. 2001-159415 discloses a loosening prevention nut that is similar to the non-loosening nut of Japanese Examined Utility Model Publication No. SHO 46-8496, where an elastic ring made of a flat-spring material etc. is attached to a top portion of the nut body. Although the nuts of Japanese Examined Utility Model Publication No. SHO 46-8496 and Japanese Unexamined. Patent Publication No. 2001-159415 include a type of flat spring for loosening prevention, they are not configured so that a pressure acts on the nut or bolt. Thus, they are not configured to prevent loosening of the threaded portions caused due to pressure.
As mentioned above, the tank device 1 is not configured to positively prevent the loosening of the threaded portions and, thus, even if it adopts the configuration of Japanese Examined Utility Model Publication No. SHO 46-8496 and Japanese Laid-open Patent Application No. 2001-159415, the loosening of the threaded portions cannot be prevented. For this reason, the valve unit 4 is threadedly coupled with the tank body 2 so that the large initial tightening torque is given to the valve unit 4 to obtain the large tightening force Fc.
For the tank device 1, the pressing force Fp that acts on the valve unit 4 becomes larger as the gas pressure in the container space 5 becomes higher, and as the pressure-receiving area of the valve unit 4 becomes larger by making an inner diameter of the opening portion 3 larger. Moreover, in a case where the tank device 1 is used in an operating condition in which the ambient temperature changes over a wide temperature variation range, an amount of decrease in the tightening force Fc due to the temperature change ΔT, that is F {f(ΔT)}, becomes greater. In such a case, when at least either one of the pressure force Fp and the amount of decrease F{f(ΔT)} becomes greater, in order to obtain the tightening force Fc that satisfies the Equation (1), a considerably large initial tightening torque is required and, thus, this operation is difficult.
Moreover, the threaded portions of the opening portion 3 and the valve unit 4 must have strength to sustain against a resultant force of the tightening force Fc and the pressure force Fp. Accordingly, for the configuration in which a large tightening force Fc is required, it is necessary to extend the dimension of the threaded portions in the axial direction to ensure a large strength of the threaded portions. Further, as the dimension of the threaded portions becomes greater, a ratio at which the opening portion 3 occupies within the tank device 1 becomes greater, and a container capacity of the container space 5 becomes less.